The ideal depth for an urban planter box growing tomatoes is usually 18–24 inches (45–60 cm). That depth gives tomato roots room to spread, buffers moisture swings, and supports staking or caging without the soil drying out too fast. If you are growing dwarf or determinate tomatoes and space is limited, 12–16 inches (30–40 cm) is the practical minimum; anything shallower makes plants struggle with heat, watering, and root competition.
Depth is only part of the plan. Choose a box at least 18–24 inches wide, with drainage holes and a lightweight, fertile soil mix. Allow one full-size tomato plant about 40–60 liters (10–15 gallons) of soil, and give indeterminate varieties more room or a larger container. Keep the soil level 1–2 inches below the rim so you can water and mulch without overflow.
In cities, also consider weight and drainage. A deep box filled with wet soil is heavy, so place it where the structure can support it and use a quality container mix rather than heavy garden soil. Check drainage holes, add a saucer or drip tray only if it does not trap water, and water deeply when the top inch or two feels dry. With 18–24 inches of depth, good drainage, and steady care, tomatoes can thrive in an urban planter box.